Process for pickling iron, steel and alloys thereof



March 19, 1957 V o. RUTHNER 5,

PROCESS FOR PICKLING IRON, STEEL AND ALLOYS THEREOF Original Filed May12, 1953 INVENTOR 10 OTHMAR RUTHNER rz l BY ATTORNEY United StatesPatent "ice PROCESS FOR PICKLING IRON, STEEL AND ALLOYS THEREOF OthmarRuthner, Vienna, Austria Continuation of application Serial No. 354,614,May 12 1953. This application November 16, 1955, Serial No. 547,220

Claims priority, application Austria February 16, 1953 15 Claims. (Cl.134-13) The present invention relates to the pickling of iron and steelor alloys thereof, and specifically to an improved process for picklingwith simultaneous reclamation of the pickling acid.

The increasing industrialization of cold-working plants placesincreasing requirements upon the pickling plants. Regenerating the spentpickling liquors can no longer be dispensed with today because theprovisions governing the purity of industrial sewage no longer toleratethe disposal of the exhausted pickling acid. Various suggestions havebeen made for the utilization and processing of the pickling acidsobtained but said suggestions were governed rather by the regulations onindustrial sewage than by technological and economical considerations.The usual procedure was to evaporate the pickling liquor, to roast thedepositing iron sulfate crystals, and, if desired, to attempt areclamation of the gases from roasting.

All known processes are subject to a substantial restriction because thepickling and the regeneration of the pickling acid constitute twooperations which are entirely separate from one another. Moreover, anumber of additional steps, viz., evaporating, crystallizing, roastingof crystals, etc., must be performed between these two operations.Though that technique has been introduced and is widely usedcommercially, it is nevertheless unsatisfactory and there is a desirefor an improved process, which is simpler from the technological aspect,and more economical.

The main object of the present invention resides in the provision of animproved process for pickling iron, steel or alloys thereof andreclaiming the pickling acid, which process is not subject to therestrictions and disadvantages of the known processes, and in theprovision of apparatus to carry out the improved process.

Another object of the invention resides in the provision of a certaincomposition and certain concentration of the pickling acid and thepickling medium.

Another important object of the invention resides in scale introducedinto the process being discharged therefrom in the form of oxide ofiron.

Further objects of the invention are: To provide apparatus for thecontinuous operation of the process for pickling and simultaneouslyreclaiming the pickling acid; to provide a pickling chamber with meansfor supplying the pickling acid and means for introducing gaseouspickling agents; to provide means for collecting and purifying thepickling liquor, means for separating the pickling liquor fromprecipitated crystals; to provide a reaction furnace for decomposing thecrystals, and to provide means to connect the several parts of theplant.

The above and further objects and the novel features of the inventionare apparent from the following description with the accompanyingdrawing, in which Fig. l and Fig. 2 schematically represent two typicalarrangements 2,785,999 Patented Mar. 19, 1957 of the several steps ofthe process and construction of the apparatus used.

Essentially the improved process for the pickling of iron, steel oralloys thereof and simultaneous reclamation of the pickling acid residesin the fact that the component steps of the process, to be described indetail hereinafter, are arranged in a novel sequence. As the firstcomponent step of the process the material to be pickled, covered withrust or scale, is pickled in a closed pickling chamber by thesimultaneous application of liquid sulfuric acid and hydrogen chloridegas. By the action of that combined pickling medium FeSO4 and FeClz areformed beside each other, and dissolve in the pickling liquor. In thepickling chamber an atmosphere is provided which contains HClso that HCltoo is taken up and dissolved in the pickling liquor. It is an essentialfeature of the process according to the invention that in the firststage of that process more Clions are taken up by the pickling liquorthan would correspond stoichiomet-rically to the FeClz. According to thelaw of mass action the solubility product can be influenced by an excessof 01- ions so that the solubility product is exceeded, FeClz becominginsoluble and crystallizing out. In the first step of the process of theinvention that reaction pattern is worked at and achieved. Whereas undernormal conditions of pickling, without an excess of Clions, FeSO4 ismore ditficulty soluble than FeClz and for this reason crystallizes outearlier, the relations are reversed according to the invention in thecase of an excess of Cl ions in the pickling liquor: From a solutioncontaining beside each other Fe++, SO4 and Clions, ferrous chlorideFeClz is crystallized out when the solution is supersaturated with HCl.

To carry out that first step of the process according to the inventionit is not only necessary to have an excess of Clions, but a certainconcentration of sulfuric acid must not be exceeded. It-has been foundthat atroom temperature the concentration of sulfuric acid should notexceed 40% H2804. With a higher content of sulfuric acid, the picklingliquor cannot dissolve enough Clions to influence the solubility of theFeClz in the desired sense.

Moreover, it was found that the process can be carried out most suitablywith a certain concentration of iron in the pickling liquor. When theproportion of the iron in solution precipitates quantitatively asferrous chloride. Expressed in grams per liter, a concentration of about70 grams per liter is preferred. The adjustment of the concentration tothe desired range can be elfected very simply by an analytical controlof the progress of the reaction. Moreover, it has been found that acertain concentration of HCl is preferable in the gas space. EX- pressedin parts by volume, that preferred ratio of air: HCl is smaller than4:]. Hence all mixtures from pure hydrogen chloride gas to a mixture ofHCl with four times its volume of air can be used as gas atmosphere inthe pickling chamber.

After the combined pickling medium described has acted on the materialto be pickled, in the first stage, of the process, the pickling liquoris supersaturated with HCl in the next step. That second step of theprocess cannot be separated from the first chronologically because oftenthe supersaturation of the solution occurs simultaneously with thepickling step. The supersaturation with HCl may be effected eitherdirectly in the pickling chamber, or in a reaction vessel which isarranged directly behind the pickling chamber and into which additionalHCl is fed. FeClz is crystallized out of the supersaturated solution andissubsequently separated from the pickling acid;

The next stage of the process relates to the separation of FeClz andH2804.

'To avoid losses of sulfuric acid in that stage of the process, theseparated ferrous chloride crystals are preferably washed withconcentrated hydrochloric acid. The pickling acid freed from thecrystals may be used again for pickling, if desired, and where theprocess is operated continuously it can be recycled to the picklingchamber. The hydrochloric acid used to wash the ferrous chloridecrystals may also be returned to the pickling process.

In the process step following the separation of the FeClz crystals thesame are decomposed thermally to form oxide of iron and HCl. Thatdecomposition may be performed in a'manner known per se in a furnace ata temperature of about 400-600 deg. C. The oxide of iron formed isdischarged from the process. The reclaimed hydrogen chloride can be fedto the pickling chamber after being cooled, to form again a combinedpickling medium in conjunction with sulfuric acid. Part of the HClreclaimed may be used to form hydrochloric acid, which in the third stepof the process serves as a washing liquid for the FeClz crystalsprecipitated from the supersaturated pickling acid.

, The process according to the invention described may bemodified toadvantage, either in its individual steps or as a whole.

It has been found that an acceleration of the pickling process can beachieved when in the first stage the hydrogen chloride is introducedinto the pickling chamber under a certain superatmospheric pressure. Inthat case the capacity of the pickling acid is increased and thesaturation' with HCl is accomplished more effectively.

With reference to the drawing Figs. 1 and 2 represent arrangements forthe continuous operation of the process according to the invention.

1- desi'gnates a closed pickling chamber With a downwardly tapering.conical bottom 2. The upper part of the chamber is provided withinjection nozzles 3, by which pickling acid is sprayed onto the material4 to be pickled. The gas supply line 5 opens into the pickling chamherwith two inlets 5' and 5", through which hydrogen chloride is blown in.The pickling acid trickles over the material to be pickled, dissolvesthe oxides of iron present in the form of scale and rust, and at thesame time is loaded with hydrogen chloride. It collects in the conicalbottom of the lower part of the pickling chamber and is supersaturatedby the hydrogen chloride entering at 5", In thelower part of thecontainer a pulp 6'is formed, whichv consists of pickling acid andcrystalline ferrous chloride. That pulp is drawn off into the centrifuge8 through the bottom outlet 7. In the centrifuge the crystals areseparated from the liquid components. The reclaimed pickling acid isreturned to the pickling chamber through line 9 and, after having wateradded thereto, if necessary, is again sprayed into the pickling chamber1 through the injection nozzles 3.

The ferrous chloride separated in the centrifuge 8 is introduced intothe oxide furnace 10' and decomposed into oxide of iron and HCl byheating. The HCl is returned into the pickling chamber afterpassing-through the cooler 11-. All reactants being retained, only FeOis discharged from the plant at 12.

Fig. 2 shows an embodiment of a continuous process, which is modified'ascompared with the arrangement described (Fig. 1). According to Fig; 2the conical bottom 2 of the reaction vessel 1 passes over into areaction column 13, into the lower part of which the gas supply line 5'discharges. The pickling chamber 1 has only one gas supply line 5 tointroduce hydrogen chloride.

The procedure in the process according to Fig. 2 has considerablesimilarity with that according to Fig. 1. A difference merely resides inthe fact that the saturation of the pickling acid with hydrogen chlorideis effected not in the picklingchamber, but in the reaction vessel 13,where also FeClz crystals are obtained. The longer path for the reactionof the acid trickling down in a countercurrent with the rising hydrogenchloride alfords advantages. if desired, the upper supply line 5 can beshut off or eliminated entirely and in this case the reaction of thehydrogen chloride gas with the pickling acid and the dissolved ironsalts substantially takes place only in the liquid phase'and only theexcess hydrogen chloride which has not been absorbed by the picklingacid enters the gas space.

The method of working described enables the pickling and regeneration tobe carried out within shorter time and at lower temperature than withthe known methods. The apparatus requires lower installation costs andless space. Moreover, the process enables substantial econom'ies asregards the expenditure of energy.

I claim: p

1. A process for the pickling of iron and steel or alloys thereof, andfor reclaiming the pickling acid, which comprises the successive steps:pickling the metal articles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl in a closed pickling chamber,supersaturating the pickling liquor with HCl to precipitate FeClzcrystals, separating the precipitated crystals from the pickling acid,and finally heating the separated crystals to decompose them into oxideof iron and HCl.

2. A process as set forth in claim 1, in which the concentration of thesulfuric acid at room temperature is less'than 40% H2804;

3. A process for the pickling of iron and steel or alloys thereof, andfor reclaiming the pickling acid, which comprises the successive steps:pickling the metal articles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl in a closed pickling chamber until aconcentratiori of about 70 grams Fe per liter has been reached,supersaturating the pickling liquor with HCl to precipitate FeClzcrystals, separating the precipitated crystals from the pickling acid,and finally heating the separated crystals to decompose them into oxideof iron and HCl.

4. A process for the pickling of iron and steel or alloys thereof, andfor reclaming the pickling acid, which comprises the successive steps:pickling the metal articles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl in a closed pickling chamber until acomposition of the pickling acid has been reached which corresponds tothe proportion supersaturating the pickling liquor with HCl toprecipitate FeClz crystals, separating the precipitated crystals fromthe pickling acid, and finally heating the separated crystals todecompose them into oxide of iron and HCl.

5. A process for the pickling of iron and steel or alloys thereof, andfor reclaiming the pickling acid, which comprises the successive steps:picklingthe metalarticles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl ina closed pickling chamber, the picklingacid being injected through injection nozzles in the upper part of thepickling chamber and conducted over the material to be pickled, andhydrogen chloride gas being introduced into the pickling chamber at thesame time to precipitate FeClz crystals, separating the precipitatedcrystals from the pickling acid, and finally heating the separatedcrystals to decompose them into oxide of iron and HCl. 7

6. A process as set forth in claim' 5, in which HCl is introduced intothe lower part of the picklin'g'chamber as well as through the bottom ofthe pickling chamber.

7. A process as claimed in claim 5 in which HCl is introduced in theupper part cf the pickling chamber and through a reaction vesselconnected to the bottom of the pickling chamber.

8. A process as claimed in claim 5, in introduced under pressure.

9. A process for the pickling of iron and steel or alloys thereof, andfor reclaiming the pickling acid, which comprises the successive steps:pickling the metal articles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl in a closed pickling chamber,supersaturating the pickling liquor with HCl to precipitate FeClzcrystals, separating the precipitated crystals from the pickling acidbelow an atmosphere having a certain HCl content, and finally heatingthe separated crystals to decompose them into oxide of iron and HCl.

10. A process for the pickling of iron and steel or alloys theerof, andfor reclaiming the pickling acid, which comprises the successive steps:pickling the metal articles with a pickling medium consisting of liquidsulfuric acid and gaseous HCl in a closed pickling chamber,supersaturating the pickling liquor with HCl to precipitate PeClzcrystals, separating the precipitated crystals from the pickling acid,washing the separated crystals with concentrated hydrochloric acid, andfinally heating the separated crystals to decompose them into oxide ofiron and HCl.

11. A continuous process for the pickling of iron and which HCl is steelor alloys thereof, and for reclaiming the pickling acid, which comprisesthe successive steps: pickling the metal articles with a pickling mediumconsisting of liquid sulfuric acid and gaseous HCl in a closed picklingchamber, supersaturating the pickling liquor with HCl to precipitateFeclz crystals, separating the precipitated crystals from the picklingacid, returning the pickling acid poor in iron into the picklingchamber, heating the separated crystals to decompose them into oxide ofiron and HCl, cooling said HCl, and finally returning said HCl to thepickling chamber for re-use as a constituent of the pickling medium.

12. A continuous process for the pickling of iron and steel or alloysthereof, and for reclaiming the pickling acid, which comprises thesuccessive steps: pickling the metal articles with a pickling mediumconsisting of liquid sulfuric acid and gaseous HCl in a closed picklingchamber, until a composition of the pickling acid has been reached whichcorresponds to the proportion supersaturating the pickling liquor withHCl to precipitate FeClz crystals, separating the precipitated crystalsfrom the pickling acid, returning the pickling acid poor in iron intothe pickling chamber, heating the separated crystals to decompose theminto oxide of iron and HCl, cooling said HCl, and finally returning saidHCl to the pickling chamber for re-use as a constituent of the picklingmedium.

13. In the cleaning of ferrous metals and their alloys in a picklingbath of sulfuric acid solution, the process of maintaining the picklingacid in an atmosphere of gaseous HCl during cleaning of the metal andsupersaturating the sulfuric acid solution with I-ICl to precipitateFeClz crystals separating the crystals from the solution and decomposingthem into iron oxide and HCl.

14. A process for pickling ferrous metals and their alloys whichcomprises immersing the metal articles to be pickled in a sulfuric acidbath maintained in an atmosphere of gaseous HCl, supersaturating theacid bath with HCl during pickling to precipitate FeClz crystals fromthe bath and maintain the strength of the sulfuric acid in the bath.

15. The process of claim 14 wherein the maximum concentration ofsulfuric acid at room temperature is No references cited.

1. A PROCESS FOR THE PICKLING OF IRON AND STEEL OR ALLOYS THEREOF, ANDFOR RECLAIMING THE PICKLING ACID, WHICH COMPRISES THE SUCCESSIVE STEPS;PICKLING THE METAL ARTICLES WITH A PICKLING MEDIUM CONSISTING OF LIQUIDSULFURIC ACID AND GASEOUS HC1 IN A CLOSED PICKLING CHAMBER,SUPERSATURATING THE PICKLING LIQUOR WITH HC1 TO PRECIPITATE FECL2CRYSTALS, SEPARATING THE PRECIPITATED CRYSTALS FROM THE PICKLING ACID,AND FINALLY HEATING THE SEPARATED CRYSTALS TO DECOMPOSE THEM INTO OXIDEOF IRON AND HC1.